Printed wiring board

ABSTRACT

The printed wiring board includes a substrate, an electrically conductive pattern formed on the substrate, and an electrical insulator covering the substrate and the electrically conductive pattern therewith, wherein a region of the electrically conductive pattern exposed through an opening formed throughout the electrical insulator is used as the pad, and a region except the pad is used as a circuit wire. The opening has opposite ends extending in a first direction beyond the electrically conductive pattern such that opposite ends of the pad in the first direction are defined by the electrically conductive pattern, and further has opposite ends extending in a second direction perpendicular to the first direction to intersect with the electrically conductive pattern such that the pad is defined in shape in the second direction by the opposite ends of the opening extending in the second direction. The pad is smaller in length in the first direction than the circuit wire.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a printed wiring board, and more particularlyto a printed wiring board including a circuit wire electricallyconnected at a distal end thereof to a pad formed on the printed wiringboard (hereinafter, simply referred to as “pad”).

2. Description of the Related Art

FIG. 7 is a plan view illustrating an example of a conventional printedwiring board.

A plurality of circuit wires 1 is arranged on a substrate in parallelwith one another. Each of the circuit wires 1 extends in a certaindirection X. Each of the circuit wires 1 has a circuit pad 34 at adistal end. An electrical insulator 3 such as a solder resist orpre-preg covers the substrate and the circuit wires 1 therewith.

The electrical insulator 3 is formed with a plurality of circularopenings 32 through each of which the pad 34 is entirely exposed.

Since the pad 34 is entirely exposed through the circular opening 32 inthe conventional printed wiring board illustrated in FIG. 7, theelectrical insulator 34 does not exist above the pad 34, and hence, thepad 34 is not compressed by the electrical insulator 3. As a result, thepad 34 makes contact with the substrate with insufficient contact force,and hence, the conventional printed wiring board illustrated in FIG. 7is accompanied with a problem that the pad 34 is likely to peel off atthe circular opening 32.

As one of solutions to the problem, the circular opening 32 may beformed smaller than the pad 34. By forming the circular opening 32smaller than the pad 34, the pad 34 is compressed at its periphery bythe electrical insulator 3, resulting in that the pad 34 makes contactwith the substrate with sufficient contact force, and hence, it ispossible to prevent the pad 34 from peeling off at the circular opening32.

However, a contact composed of solder or paste containing electricalconductor, formed on the pad 34, makes contact only with a surface ofthe pad 34, and does not make contact with a sidewall of the pad 34;resulting in another problem of insufficient contact force between thepad 34 and a contact formed on the pad.

Furthermore, since the circular pad 34 is formed at a distal end of thecircuit wire 1 in the conventional printed wiring board illustrated inFIG. 7, a pitch between adjacent pads 34 is unavoidably small. An areaarray type LSI package most often used presently is designed to have alot of terminals at a small pitch in accordance with a need thatelectronic devices are mounted at a high density. Thus, when wires areconnected to the terminals of such an area-array type LSI package, itwould be quite difficult to increase the number of wires extendingthrough a gap between the adjacent pads.

Japanese Utility Model Application No. 63-126428 has suggested a printedwiring board including an electrically insulating substrate, a pluralityof pads formed on the substrate, and solder resist thicker than anelectrical conductor covering the pads at ends therewith, wherein thepads have a width greater than a width of the solder resist.

Japanese Patent Application Publication No. 8-64939 has suggested aprinted wiring board on which a device including a plurality of elongateterminals is to be mounted, including a substrate, a plurality of solderpads arranged on an upper surface the substrate for soldering theterminals therewith, and solder resist formed on the substrate in anarea except the solder pads for preventing adhesion of molten solder.The substrate is formed on a lower surface with a region in which solderresist is not formed for absorbing extra molten solder.

Japanese Patent Application Publication No. 8-107264 has suggested aprinted wiring board including a substrate, a terminal formed on thesubstrate, and an electrically insulating layer covering the terminaltherewith. The electrically insulating layer is formed with an openingthrough which a principal surface area of the substrate except an edgeof the terminal is exposed, and is thicker than the terminal.

Japanese Patent Application Publication No. 9-27661 has suggested aprinted wiring board on which a plurality of electrodes is arranged at apredetermined pitch. Each of the electrodes is comprised of a main bodyhaving a certain shape, and an extended portion wider than the main bodyin a first direction in which the electrodes are arranged. The extendedportions of the electrodes disposed adjacent to each other are spacedaway from each other in a second direction perpendicular to the firstdirection.

Japanese Patent Application Publication No. 2000-77471 has suggested asubstrate on which an electrically conductive pattern on which a bump ofan electronic device is mounted in flip-chip is formed. The electricallyconductive pattern is comprised of a wire pattern which will make awire, and a pad formed integrally with the wire pattern at a location atwhich the bump is mounted. The electrically conductive pattern has awidth W2 greater than a width W1 of the pad.

SUMMARY OF THE INVENTION

In view of the above-mentioned problems in the conventional printedwiring board, it is an object of the present invention to provide aprinted wiring board having a pad structure which is capable ofproviding sufficient contact force between a pad and a substrate andsufficient contact force between a pad and a contact.

It is another object of the present invention to provide a printedwiring board having a pad structure making it possible to arrange padsat an extended pitch.

Hereinbelow is described the printed wiring board in accordance with thepresent invention through the use of reference numerals used in laterdescribed embodiments. The reference numerals are indicated only for thepurpose of clearly showing correspondence between claims and theembodiments. It should be noted that the reference numerals are notallowed to interpret of claims of the present application.

The present invention provides a printed wiring board including asubstrate (12), an electrically conductive pattern formed on thesubstrate (12), and an electrical insulator (3) covering the substrate(12) and the electrically conductive pattern therewith, wherein a regionof the electrically conductive pattern exposed through an opening (2)formed throughout the electrical insulator (3) is used as a pad (4), anda region except the pad (4) is used as a circuit wire (1), the opening(2) is formed so as to separately expose the electrically conductivepattern, the opening (2) has opposite ends (22) extending in a first (Y)direction beyond the electrically conductive pattern such that oppositeends of the pad (4) in the first (Y) direction are defined by theelectrically conductive pattern, the opening (2) further has oppositeends (21) extending in a second (X) direction perpendicular to the first(Y) direction to intersect with the electrically conductive pattern suchthat the pad (4) is defined in shape in the second (X) direction by theopposite ends (21) of the opening (2) extending in the second (X)direction, and the pad (4) is smaller in length in the first (Y)direction than the circuit wire (1).

The present invention further provides a printed wiring board includinga substrate (12), an electrically conductive pattern formed on thesubstrate (12), and an electrical insulator (3) covering the substrate(12) and the electrically conductive pattern therewith, wherein a regionof the electrically conductive pattern exposed through an opening (2)formed throughout the electrical insulator (3) is used as a pad (4), anda region except the pad (4) is used as a circuit wire (1), the opening(2) is formed so as to separately expose the electrically conductivepattern, the opening (2) has opposite ends (22) extending in a first (Y)direction beyond the electrically conductive pattern such that oppositeends of the pad (4) in the first direction are defined by theelectrically conductive pattern, the opening (2) further has oppositeends (21) extending in a second (X) direction perpendicular to the first(Y) direction to intersect with the electrically conductive pattern suchthat the pad (4) is defined in shape in the second (X) direction by theopposite ends (21) of the opening (2) extending in the second (X)direction, and the pad (4) is smaller in length in the first (Y)direction than the circuit wire (1), the pad (4) is comprised of aplurality of pads (4C) arranged in the second (X) direction, the circuitwire (1) is comprised of a plurality of circuit wires spaced away fromone another in the first (Y) direction and extending in the second (X)direction, and at least one of the circuit wires (1) is bending byhaving a component directed toward the first (Y) direction, and iselectrically connected to the pad (4) at a side (4 a) thereof extendingin the second (X) direction.

The pad (4) may be comprised of a plurality of pads (4C) arranged in thesecond direction as well as in the first direction, in which case, thecircuit wire (1) is arranged for each of pad rows arranged in the seconddirection.

It is preferable that the electrically conductive pattern has oppositeends (6) in the second direction, the opening (2) is spaced away fromthe opposite ends (6) of the electrically conductive pattern, and thecircuit wire (1) is electrically connected to the pad (4) at a side (4a) thereof extending in the second direction.

The advantages obtained by the aforementioned present invention will bedescribed hereinbelow.

The first advantage provided by the present invention is that a pad canbe made smaller, and it is possible to prevent reduction in a contactforce between a pad and a substrate and a contact force between a padand an electrical conductor.

This is because a pad can have a region in which the pad is covered withan electrical insulator and a region in which the pad is not coveredwith an electrical insulator.

The second advantage provided by the present invention is that a printedwiring board on which area-array LSI packages are to be mounted can befabricated in low costs in a conventional process.

This is because that the printed wiring board in accordance with thepresent invention makes it possible to increase a gap formed betweenadjacent pads, ensuring that the number of circuit wires to be extendedthrough the gap can be increased, and thus, circuit wires extending fromterminals of an area-array LSI package can be arranged on a surface ofthe printed wiring board.

Thus, it would be possible to reduce the number of layers of the printedwiring board, preventing an increase in costs for fabrication of theprinted wiring board.

The above and other objects and advantageous features of the presentinvention will be made apparent from the following description made withreference to the accompanying drawings, in which like referencecharacters designate the same or similar parts throughout the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a partial plan view of the printed wiring board in accordancewith the first embodiment of the present invention.

FIG. 2(a) is an enlarged plan view of the printed wiring board inaccordance with the first embodiment of the present invention, FIG. 2(b)is a cross-sectional view taken along the line B-B in FIG. 2(a), andFIG. 2(c) is a cross-sectional view taken along the line C-C in FIG.2(a).

FIG. 3 is a partial plan view of the printed wiring board in accordancewith the second embodiment of the present invention.

FIG. 4 is a partial plan view of the printed wiring board in accordancewith the third embodiment of the present invention.

FIG. 5 is a partial plan view of the printed wiring board in accordancewith the fourth embodiment of the present invention.

FIG. 6 is a partial plan view of the printed wiring board in accordancewith the fifth embodiment of the present invention.

FIG. 7 is a partial plan view of an example of a conventional printedwiring board.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments in accordance with the present invention will beexplained hereinbelow with reference to drawings.

First Embodiment

FIG. 1 is a partial plan view of a printed wiring board 10 in accordancewith the first embodiment of the present invention. FIG. 2(a) is anenlarged plan view of the printed wiring board 10, FIG. 2(b) is across-sectional view taken along the line B-B in FIG. 2(a), and FIG.2(c) is a cross-sectional view taken along the line C-C in FIG. 2(a).

The printed wiring board 10 in accordance with the first embodiment iscomprised of a substrate 12 (see FIG. 2(b)), four circuit wires 1 formedon the substrate 12, and electrical insulator 3 covering the substrate12 and the circuit wires 1 therewith.

The four circuit wires 1 extend in X direction (second direction) inparallel with one another, and are spaced away from one another in Ydirection (first direction) perpendicular to the X direction. Each ofthe circuit wires 1 is formed as an electrically conductive patternformed by patterning electrically conductive material deposited on thesubstrate 12.

The electrical insulator 3 is formed with a plurality of rectangularopenings 2. A part of the circuit wire 1 exposed through the opening 2is used as a pad 4. That is, among the four circuit wires 1 in theprinted wiring board 10 in accordance with the first embodiment, each ofthe electrical conductive patterns located uppermost and lowermost iscomprised of the circuit wire 1 extending in the X direction and the pad4 electrically connected to the circuit wire 1, and each of theintermediate two electrically conductive patterns is comprised of thecircuit wire 1 extending in the X direction.

The pad 4 is in the form of a rectangle having the same width (length inthe Y direction) as that of the circuit wire 1. Since the pad 4 is notcircular unlike the pad 34 in the conventional printed wiring boardillustrated in FIG. 7, it is possible to arrange the pads 4 disposedadjacent in the Y direction, at a greater pitch. Thus, two circuit wires1 can be arranged between the uppermost and lowermost circuit wires.

Opposite ends 22 of the opening 2 in the Y direction extend outwardlybeyond the circuit wire 1. Hence, sidewalls of the pad 4 in the Ydirection are exposed. Thus, electrical conductor to be deposited on thepad 4 covers not only a surface of the pad 4, but also sidewalls of thepad 4 therewith, ensuring enhancement in contact force between the pad 4and the electrical conductor.

In contrast, opposite ends 21 of the opening 2 in the X direction extendacross the circuit wire 1. Hence, the pad 4 is compressed by theelectrical insulator 3 at the opposite ends 21. As a result, the pad 4makes contact with the substrate 12 with increased contact force,ensuring that the pad 4 is hardly peeled off the substrate 12.

Hereinbelow is explained the increased contact force between the pad 4and, the substrate 12, with reference to FIG. 2.

As illustrated in FIG. 2(a), the circuit wire 1 comprised of anelectrically conductive pattern is formed on the substrate 12. The pad 4defined by the opening 2 formed with the electrical insulator 3 isformed continuously with the circuit wire 1. As illustrated in FIG.2(b), a connection bump electrode as an electrical conductor 11 isformed on the pad 4. The pad 4 is electrically connected to a LSIpackage pad 13 formed on a lower surface of a LSI package 14, throughthe electrical conductor 11.

As mentioned above, since the opposite ends 22 of the opening 2 in the Ydirection are spaced away from the pad 4, the electrical conductor 11spreads over the pad 4 in wet condition, and further spreads tosidewalls of the pad 4, as illustrated in FIG. 2(b). Thus, theelectrical conductor 11 surrounds the pad 4, preventing reduction in acontact force between the pad 4 and the electrical conductor 11.

On the other hand, the pad 4 is covered with the electrical insulator 3at the opposite ends 21 of the opening 2 in the X direction, asillustrated in FIG. 2(c). Thus, the pad 4 is compressed by theelectrical insulator 3. As a result, a force for preventing the pad 4from peeling off the substrate 12 acts on the pad 4, preventingreduction in a contact force between the pad 4 and the substrate 12.

In the first embodiment, since a length W1 of the pad 4 in the Ydirection is equal to a length W2 of the circuit wire in the Ydirection, it is possible to fabricate the pad 4 extending from theelectrically conductive pattern having the same width and electricallyconnected to the circuit wire 1.

The printed wiring board 10 in accordance with the first embodiment isdesigned to include four circuit wires 1. However, the number of thecircuit wires 1 is not to be limited to four. Any integer equal to orgreater than one may be selected.

Furthermore, a shape of the opening 2 of the electrical insulator 3 isnot to be limited to a rectangle. The opening may be in the form of anyquadrangle such as a parallelogram or a rhombus.

Second Embodiment

FIG. 3 is a partial plan view of a printed wiring board 20 in accordancewith the second embodiment of the present invention. Parts or elementsin FIG. 3 that correspond to those illustrated in FIGS. 1 or 2 have beenprovided with the same reference numerals, and are not explained.

The printed wiring board 20 in accordance with the second embodiment isdifferent from the printed wiring board 10 in accordance with the firstembodiment in a shape of a pad. That is, in the second embodiment, awidth (a length in the Y direction) of the opening 2 of the electricalinsulator 3 is designed equal to a width of the circuit wire 1, and awidth of the pad is designed smaller than a width of the opening 2 ofthe electrical insulator 3 in order to narrow a distance betweenadjacent circuit wires 1.

Specifically, as illustrated in FIG. 3, a length W1 of a pad 4A in the Ydirection is designed smaller than a length W2 of the circuit wire 1 inthe Y direction.

The second embodiment is useful in a case that the pad 4A can makecontact with the electrical insulator 11 in a sufficiently large contactarea, but it is necessary to make further spatial margin betweenadjacent circuit wires relative to expansion of the electrical conductor11 in the Y direction (see FIG. 2(b)).

Third Embodiment

FIG. 4 is a partial plan view of a printed wiring board 30 in accordancewith the third embodiment of the present invention. Parts or elements inFIG. 4 that correspond to those illustrated in FIGS. 1 or 2 have beenprovided with the same reference numerals, and are not explained.

The printed wiring board 30 in accordance with the third embodiment isdifferent from the printed wiring board 10 in accordance with the firstembodiment in a shape of a pad.

Specifically, as illustrated in FIG. 4, a length W1 of a pad 4B in the Ydirection is designed greater than a length W2 of the circuit wire 1 inthe Y direction.

The third embodiment is useful in a case that there is sufficientspatial margin between adjacent circuit wires, but it is necessary toincrease a contact area through which the pad 4B makes contact with theelectrical insulator 11.

Fourth Embodiment

FIG. 5 is a partial plan view of a printed wiring board 40 in accordancewith the fourth embodiment of the present invention. Parts or elementsin FIG. 5 that correspond to those illustrated in FIGS. 1 or 2 have beenprovided with the same reference numerals, and are not explained.

The printed wiring board 40 in accordance with the fourth embodiment isdifferent from the printed wiring board 10 in accordance with the firstembodiment in a direction in which the circuit wire 1 is connected tothe pad 4.

In the printed wiring board 10 in accordance with the first embodiment,the; circuit wire 1 and the pad 4 are arranged in a common line. Incontrast, in the fourth embodiment, the circuit wire 1 is connected to apad 4C at a sidewall 4a of the pad 4C extending in the X direction, asillustrated in FIG. 5.

The fourth embodiment is useful in a case that it is necessary to directthe circuit wire 1 and the pad 4C differently from each other.

A relation between a length L from a distal end 6 of the circuit wire 1to an end 21 of the opening 2 in the X direction and a length W1 of thepad 4C in the:Y direction is identical with the same in the firstembodiment.

Fifth Embodiment

FIG. 6 is a partial plan view of a printed wiring board 50 in accordancewith the fifth embodiment of the present invention. Parts or elements inFIG. 6 that correspond to those illustrated in FIG. 5 have been providedwith the same reference numerals, and are not explained.

The fifth embodiment corresponds to a variation of a combination of thefirst and fourth embodiments.

In the printed wiring board 50 in accordance with the fifth embodiment,illustrated in FIG. 6, the pads 4C are arranged in four rows in the Xdirection, and the circuit wires 1 extend from the pads in a commondirection (to the right in FIG. 6). Among the pads 4C in four rows, inthe pad 4C located at the left end, the circuit wire 1 is connected tothe pad 4C at a sidewall 4a of the pad 4C extending in the X direction,similarly to the fourth embodiment illustrated in FIG. 5. Among the pads4C in four rows, in the rest of the pads 4C, the circuit wires 1 and thepads 4C extend in a common line.

A dashed line around each of the pads 4C is an imaginary line 24indicative of an outline of the conventional circular pad 34 (see FIG.7).

When the circuit wire 1 extends from the conventional pad 34, thecircuit wire 1 may intersect with the pad 34. For instance, when thecircuit wire 1 extends from the pad 4C located in an upper row and atthe left end, the circuit wire 1 intersects with the pads 34 (that is,the imaginary line 24) located in a lower row and at the second tofourth from the left end. When the circuit wire 1 extends from the pad4C located in an upper row and at the second from the right end, thecircuit wire 1 intersects with the pad 34 located in an upper row and atthe Bright end.

Thus, the number of the circuit wires 1 which can be extended from thepads 34 in the conventional printed wiring board is smaller than thesame in the fifth embodiment. As a result, it would be necessary forinstance to arrange the circuit wires 1 in inner layers throughvia-holes, resulting in complexity of a structure of the printed wiringboard. In the fifth embodiment, since the opening 2 of the electricalinsulator 3 is designed to be rectangular, it would be possible to avoidcomplexity in a structure of the printed wiring board.

Hereinbelow, explanation is made with reference to an example.

If a pad is formed smaller in an area, a contact area between a pad andthe electrical conductor 11 would be reduced accordingly. Hence, inorder to enhance electrical characteristics, it is preferable that alength of the opening 2 in the X direction, which exerts an influence onan area of a pad, and a length of the circuit wire 1 are designed aslong as possible, taking into consideration a gap between the distalends 6 of the circuit wires 1 located adjacent to each other in the Xdirection.

For instance, in an area-array LSI package 13 having a pitch of 0.8 mm,a pad diameter is usually in the range of 0.35 mm to 0.45 mm bothinclusive, and a gap between adjacent pads is in the range of 0.35 mm to0.45 mm both inclusive.

When wires each having a width of 75 micrometers and spaced from oneanother at a pitch of 75 micrometers are arranged through theabove-mentioned gap ranging from 0.35 mm to 0.45 mm, the number of thewires which can be arranged through the gap is 1 or 2. Thus, if three ormore wires are arranged through the gap, it would be necessary to designa printed wiring board to have multi-layered structure, and to arrangewires in inner layers and extend the wires through via-holes.

When the rectangular pad 4C in the fifth embodiment is used, if thecircuit wire has a width of 75 micrometers, the pad 4C would have awidth of 75 micrometers, and hence, a gap between the adjacent pads 4Cwould be 0.725 mm. Thus, the number of the circuit wires 1 which can beextended through the gap is four, ensuring an increase in the number oflayers in the printed wiring board in comparison with the conventionalprinted wiring board having the circular pads 34.

While the present invention has been described in connection withcertain preferred embodiments, it is to be understood that the subjectmatter encompassed by way of the present invention is not to be limitedto those specific embodiments. On the contrary, it is intended for thesubject matter of the invention to include all alternatives,modifications and equivalents as can be included within the spirit andscope of the following claims.

The entire disclosure of Japanese Patent Application No. 2002-143530filed on May 17, 2002 including specification, claims, drawings andsummary is incorporated herein by reference in its entirety.

1. A printed wiring board comprising: a substrate; an electrically conductive pattern formed on said substrate; and an electrical insulator covering said substrate and said electrically conductive pattern therewith, wherein a region of said electrically conductive pattern exposed through an opening formed throughout said electrical insulator is used as a pad, and a region except said pad is used as a circuit wire, said opening is formed so as to separately expose said electrically conductive pattern, said opening has opposite ends extending in a first direction beyond said electrically conductive pattern such that opposite ends of said pad in said first direction are defined by said electrically conductive pattern, said opening further has opposite ends extending in a second direction perpendicular to said first direction to intersect with said electrically conductive pattern such that said pad is defined in shape in said second direction by said opposite ends of said opening extending in said second direction, and said pad is smaller in length in said first direction than said circuit wire.
 2. A printed wiring board comprising: a substrate; an electrically conductive pattern formed on said substrate; and an electrical insulator covering said substrate and said electrically conductive pattern therewith, wherein a region of said electrically conductive pattern exposed through an opening formed throughout said electrical insulator is used as a pad, and a region except said pad is used as a circuit wire, said opening is formed so as to separately expose said electrically conductive pattern, said opening has opposite ends extending in a first direction beyond said electrically conductive pattern such that opposite ends of said pad in said first direction are defined by said electrically conductive pattern, said opening further has opposite ends extending in a second direction perpendicular to said first direction to intersect with said electrically conductive pattern such that said pad is defined in shape in said second direction by said opposite ends of said opening extending in said second direction, and said pad is smaller in length in said first direction than said circuit wire, said pad is comprised of a plurality of pads arranged in said second direction, said circuit wire is comprised of a plurality of circuit wires spaced away from one another in said first direction and extending in said second direction, and at least one of said circuit wires is bending by having a component directed toward said first direction, and is electrically connected to said pad at a side thereof extending in said second direction.
 3. The printed wiring board as set forth in claim 2, wherein said pad is comprised of a plurality of pads arranged in said first and second directions, and said circuit wire is arranged for each of pad rows arranged in said second direction.
 4. The printed wiring board as set forth in any one of claims 1 to 3, wherein said electrically conductive pattern has opposite ends in said second direction, said opening is spaced away from said opposite ends of said electrically conductive pattern, and said circuit wire is electrically connected to said pad at a side thereof extending in said second direction. 